The present invention relates generally to control valves of the type commonly utilized in vehicular power steering systems and, more particular, to such control valves having one or more parasitic leakage orifices provided for optimizing the performance characteristic of the power steering system.
It is known that power steering systems equipped with control valves having minimal control orifice area are capable of generating significant hydraulic "assist" at low, or even zero, input deflection angles. Such capability is demonstrated from the performance characteristics for the low speed section of the power steering valve described in SAE Paper 880707, and entitled 1988 LINCOLN CONTINENTAL VARIABLE-ASSIST POWER STEERING SYSTEM. However, the presence of significant hydraulic assist at zero input deflection angle typically results in unacceptably light "on-center" feel. As further described in the SAE paper, the steering system includes a parallel flow circuit having a conventionally sized orifice area which is progressively activated at higher speeds. Utilization of such a parallel flow circuit results in minimal hydraulic assist at low, or even moderate, input deflection angles. Unfortunately, a side effect of eliminating virtually all hydraulic assist at low input deflection angles is a significant reduction in steering precision. Under such condition, the vehicle will tend to "wander" when subjected to transient conditions, such as those caused by wind loadings.
Modernly, the ability to provide variable hydraulic assist with respect to vehicular speed (i.e., commonly referred to as "speed sensitive" steering) is required in most power steering systems. A conventional method of providing speed sensitive steering is achieved by selectively varying the fluid supply flow rate to an otherwise standard control valve. This is usually accomplished via the incorporation of an EVO (electronically variable orifice) valve into the flow control circuit of the host system's fluid supply pump. As a result, a reduction in the fluid supply flow rate at higher vehicle speeds causes a lowered hydraulic flow gain function for the control valve. Thus, at very low vehicle speeds the amount of fluid flow through the control valve is maximized for reducing sterring effort (i.e., for parking requirements). At higher vehicle speeds, as the rate of fluid flow through the control valve is reduced, any given amount of hydraulic steering assist requires a concomitantly greater input deflection angle. Since this requires rotational movement of the steering wheel against the restoring force of a spring member (i.e., a torsion bar), more input torque is required such that steering forces are increased when the motor vehicle is being driven on the highway. Again however, the unwanted side effects of excessive valve deflection and a reduction in steering precision are present.
In the event that rapid steering wheel motion is required when the power steering system is subjected to the reduced flow rates (i.e., an "accident avoidance maneuver"), there may be insufficient fluid flow to adequately provide for the concomitant displacement of the system's power cylinder. This undesirable condition is commonly referred to as "pump catch" whereby the vehicle operator momentarily encounters a sharp increase in resistance to steering motion. Accordingly, means must also be provided for sensing rapid motions of the steering wheel and instantaneously modifying the EVO valve setting in order to increase flow to avoid the occurrence of "pump catch". Unfortunately, this leads to another unwanted side effect wherein steering effort is suddenly diminished during the "accident avoidance maneuver", which may potentially exacerbate the steering control difficulties already being encountered during the "accident avoidance maneuver".
Accordingly, the present invention is directed to providing a vehicular power steering system having an "open-center" control valve that has substantially zero hydraulic flow gain at zero input deflection angle, and yet has minimal control orifice area. This valving arrangement is desirable in that it leads to a rapid restoration of hydraulic flow gain at relatively small input deflection angles.
In a first preferred embodiment, an improved valving sub-assembly is provided having small transverse leakage slots formed in a valve spool that are positioned in parallel with either input or return flow distribution slots also formed in the valve spool. The leakage slots are configured to form parasitic flow orifices which fluidically interconnect output slots formed in a valve sleeve. In this manner, a selectively variable "leak" is provided across the power cylinder. As a result, the relationship between output pressure and valve deflection at, or near, zero valve deflection values is of a second order such that an output pressure slope having substantially zero value at zero valve deflection is generated. Moreover, even though hydraulic flow gain is substantially zero at zero valve deflection, a smooth recovery to normal values at modest values of valve deflection is generated for providing optimal static control characteristics for the power steering control valve.
As a related feature, since the parasitic flow orifices allow utilization of minimally sized main flow control orifices, the characteristic "output pressure input deflection angle" curves are relatively widely separated as a function of load flow rates. This relationship is indicative of the presence of relatively large amounts of input torque modulation as rotational velocity inputs are made to the steering wheel. Therefore, the host power steering system "feels" highly damped and is very smooth in operation. In addition, a relatively stiff torsion bar can be utilized within the control valve, thus yielding increased steering precision steering wherein extremely small values of positional error (i.e., due to valve deflection) are present.
An alternate design philosophy is incorporated into yet another preferred embodiment of the present invention wherein first and second sets of parasitic slots are arranged such that their corresponding parasitic orifices close sequentially with respect to the input and return flow control orifices. More particularly, both sets of parasitic orifices are open at zero valve deflection. However, the first set of parasitic orifices closes at a smaller valve deflection than the input and return flow control orifices and the second set of parasitic orifices closes at a larger valve deflection than the input and return flow control orifices. This results in a greater range of design freedom for the valve designer in shaping the various output pressure-valve deflection characteristic curves as a function of load flow.
Other features, objects and advantages of the present invention will become readily apparent to one skilled in the art upon analysis of the following written description taken in conjunction with the accompanying drawings and appended claims.